The relation between melanism and thermal biology in a colour polymorphic bush cricket, Isophya rizeensis

According to the thermal melanism hypothesis, darker coloured melanic individuals heat up faster and to higher temperatures than lighter coloured individuals due to lower skin reflectance. Consequently, it is assumed that darker melanic types may be advantageous compared to light coloured types in colder regions. As temperature gradually decreases with elevation and latitude the degree of melanism is expected to increase along these gradients in ectothermic species. Isophya rizeensis, a colour polymorphic bush cricket species endemic to Northeastern Turkey is an interesting case since the degree of melanism decreases with elevation, contrary to the thermal melanism hypothesis. In order to investigate the relation between colouration and thermal biology of this species, body temperatures (Tb) of crickets from different colour morphs, environmental temperatures (Ta), solar radiation and vegetation height were measured to test the relation between these variables and thermoregulation. Field results showed that solar radiation was the most effective factor on temperature excess (Tex), the difference between body and ambient temperature. Additionally, Tex values showed negative correlation with vegetation height. Although Tex values did not differ significantly between colour morphs, paired experiments under sunlight showed that darker morphs heated up faster and attained higher body temperatures than light morphs. We conclude that, since higher Tex values at alpine short swards might also increase the risk of facing deleterious temperatures at high elevations, protection against overheating might be one of the factors responsible for this polymorphism.